The invention relates to a lighting system, comprising at least one light source for supplying artificial light and a control unit for controlling the light source, the control unit comprising means for forming a control signal which is dependent on the daylight level. The invention also relates to a control unit for use in such a lighting system.
A lighting system of this kind is widely used, notably for the lighting of office buildings. In known systems the means for forming a control signal which is dependent on the daylight level generally comprise a light sensor for measuring the daylight level. The control unit is then arranged to switch on the artificial light when the measured daylight level drops below a predetermined minimum or, conversely, to switch off the artificial light when the measured daylight level exceeds a predetermined maximum. Systems of this kind are also known as street lighting systems. It is known in particular that in office lighting systems the control unit adjusts the intensity of the artificial light mainly inversely proportionally to the level of the daylight.
A large-scale study has revealed that for 85% of the office workers good lighting highly contributes to office comfort Harris Louis: Office lighting, comfort and productivity-how the workers feel. Lighting Design and Application No. 10, July 1980!. It is known that in this respect light plays a visual as well as a non-visual role. As regards the visual role, it is important, evidently, that the appropriate amount and type of lighting are used to perform a given task. As regards the non-visual role it is known that various processes within the human body are influenced by light. Examples of such processes are the 24-hour rhythm (circadian rhythm) of the sleeping-activity cycle and of the production of some hormones. The non-visual aspects of light, consequently, have an indirect effect on the performance and effectiveness of humans.
The foregoing emphasizes the important role of light. In many environments, such as offices, factories but also living rooms, light is formed by a combination of incident daylight and added artificial light. In many cases the daylight cannot be influenced, or only to a limited extent, by the user, for example by opening or closing a blind. This makes control of the artificial light all the more important.
It is an object of the invention to provide a lighting system of the kind set forth which takes into account human preferences.
To this end, the lighting system in accordance with the invention is characterized in that the light source is of the type having an adjustable colour temperature, and the control unit is arranged to adjust the colour temperature of the light source in dependence on a predetermined relationship between the daylight level and the colour temperature of the artificial light.
The invention is based on the insight, gained by tests, that test persons demonstrate a preference for a given colour temperature of the artificial light, which colour temperature is dependent on the intensity of the incident daylight. For example, by utilizing a light sensor for measuring the level of the incident daylight, the control unit can adjust the colour temperature of the artificial light in dependence on the measured daylight level.
An embodiment of the lighting system in accordance with the invention is characterized in that the means for forming the control signal comprise a day calendar unit for determining the day of the year and are arranged to form the control signal in dependence on a predetermined relationship between the day of the year and the mean daylight level. In a simple version of this embodiment the daylight level is estimated while utilizing a day calendar unit for determining the day of the year. On the basis of a predetermined relationship between the day of the year and the mean daylight level, the daylight level can be estimated so as to be used to adjust the colour temperature.
A less simple version of said embodiment is characterized in that the means for forming the control signal also comprise a clock and are arranged to form the control signal in dependence on a predetermined relationship between on the one hand the day of the year and the time of day and on the other hand the mean daylight level. As a result of the use of a clock, the daylight level at any time of day can be simply estimated better, resulting in a better adjustment of the colour temperature of the artificial light.
An even more advanced version of said embodiment is characterized in that the means for forming the control signal also comprise a light sensor for measuring the actual daylight level, that the control unit stores at least two different, predetermined relationships between the daylight level and the colour temperature, and that the control unit is arranged to select one of said relationships in dependence on the measured actual daylight level. For example, by storing different relationships for different types of weather, such as clear, overcast or mixed, and by selecting the most appropriate relationship on the basis of the measured daylight level, an even better adjustment of the colour temperature of the artificial light is achieved.
An embodiment of the lighting system in accordance with the invention is characterized in that the control unit stores at least two different, predetermined relationships between the daylight level and the colour temperature, and that the control unit comprises a first control member which is arranged to select one of said relationships. Human tastes, generally speaking, are very diverse. This also becomes apparent in the form of different preferences for light settings. Some people prefer "warmer" light whereas others prefer "cooler" light. In order to satisfy these various preferences in a simple manner, the latter embodiment of the system offers the user a selection from at least two predetermined relationships.
An embodiment of the lighting system in accordance with the invention is characterized in that the control unit comprises modification means which are arranged to modify the predetermined relationship between the daylight level and the colour temperature. In order to comply even better with the user's preferences, this embodiment of the system offers the possibility of modification of the predetermined relationship. Like in the foregoing embodiment, on the one hand this enables optimization of the control system for a given office building, for example taking into account the situation and general layout of the building. On the other hand, if the offices can be individually controlled, per office a relationship can thus be adapted to the individual wishes of the user. An improved version of this embodiment of the lighting system in accordance with the invention is characterized in that the control unit comprises a second control member which is arranged to readjust the adjusted colour temperature and to operate the modification means. As opposed to the foregoing embodiments, where the user influences the control only indirectly by selection or modification of a relationship, in this embodiment the user can readjust the colour temperature directly. On the basis of this readjustment, the system also modifies the desired relationship between the daylight level and the colour temperature. The individual preferences of persons can thus be satisfied even better.
An embodiment of the lighting system in accordance with the invention is characterized in that the predetermined relationship between the daylight level and the colour temperature of the artificial light, adjusted by the user, constitutes mainly an increase of the colour temperature as the daylight level increases. Tests have shown that a positive correlation exists between the daylight level and the colour temperature of the artificial light, so that a lighting system satisfying these requirements can satisfy the wishes of the average user.
An embodiment of the lighting system in accordance with the invention is characterized in that the predetermined relationship between the daylight level and the colour temperature of the artificial light means that when the daylight level, measured on an office desk, increases from a proximately 400 lux to approximately 1800 lux, the colour temperature increases from approximately 3300 K to approximately 4300 K. Tests have demonstrated that such a relationship is a suitable representation of the wishes of the average test person. A lighting system utilizing such a relationship as a basis can highly satisfy user wishes concerning the adjustment of the colour temperature.
These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter.